#app4 .circle{
    width: 80px;
    height: 80px;
    border-radius: 50%;
    border: 1px solid grey;
}

@keyframes change{
    0%{
        background-color: red;
    }
    100%{
        background-color: green;
    }
}

#app4 .circle.active{
    animation: change 2s infinite alternate linear;
}